The invention relates to a doctor rod for a coating device, especially for a device for the coating of paper or cardboard webs with pigmented coloring agents, which have a cylindrical base body of steel onto whose outer surface a hard material layer of chromium or ceramic is applied.
In applicator and metering systems of apparatus for the coating of paper or cardboard webs, doctor bars are used in a known manner as metering elements. They serve to produce a precisely metered film of coating material (glue, sizing or starch, pigmented coloring agents, etc.) on a roller or directly on a paper or cardboard web by doctoring off the excess which has been applied by an applicator unit upstream thereof.
In the coating of paper or cardboard webs, doctor bars with a diameter between 8 mm and 50 mm are used and can extend over the entire working width and which can thus have a length up to 10 m. So that the doctor bar is supported over its entire length against sagging, it is usually supported in a bed of an elastomeric material which has an opening turned toward the roller or web and from which the periphery of the doctor bar projects. A doctor bar supported in this manner is described in DE 30 22 955-C.
The doctor bars either have a smooth outer surface (EP 0 454 643-B1) or are profiled on their outer surface with circumferential grooves (DE 37 35 889-A1). With doctor bars with profiled outer surfaces, the coating thickness of the metered film on the roller/web can be influenced by the free cross sectional areas of the grooves. They are preferably used in indirect applicator processes to apply a metered film of the coating material onto a roller which then transfers the coating material onto a web in a pressing gap (EP 0 453 427-A2).
Since the doctor bars, especially in application of pigment coloring agent coatings which have abrasive effects, are highly stressed in the wear sense, it is known to form them from a cylindrically shaped base body of steel which is coated with a hard material layer of chromium (EP 0 454 643-B1) or of ceramic (DE 39 37 749-A1). Even doctor bars provided with a hard material layer of chromium or ceramic wear in use so that they must be replaced at regular time intervals.
The object of the invention is to provide a doctor bar for a coating apparatus which has a greater useful life by comparison with the known doctor bars.
This object is attained with a coating apparatus which has a cylindrical base body of steel and on whose outer surface a hard material layer of chromium or ceramic is applied, wherein the cracks and craters in the surface of the hard material layer are closed by a compound produced in an ion implantation process by the reaction of the hard material with ions applied in a plasma.
It has been found that in the application of the hard material layer to the base body of steel, microcracks and microcraters arise in its surface and negatively effect the wear resistance and corrosion characteristics of the doctor bar. Additionally, the surface finishing of the invention increases the hardness of the doctor bar. A further advantage is that the coating quality of the films metered onto a surface by the doctor bar can be improved since the surface of the doctor bar has a reduced roughness. The coating is thus more uniform.
According to the invention, the hard material surface is treated with an ion mixture in the production of the doctor bar which contains nitrogen and/or carbon atoms but does not contain any nonmetal ions. The covering compound can be composed of chromium nitride, chromium carbide and/or chromium carbonitride. A chromium layer of 20 xcexcm to 60 xcexcm, preferably 40 xcexcm to 50 xcexcm, in thickness can be applied as the hard material layer. The chromium layer after the plasma treatment can have a hardness in excess of 1200 HV, preferably from 1400 HV to 1600 HV. The outer surface of the doctor bar has circumferentially extending grooves. In a central longitudinal section through the doctor bar the open cross sectional area of the grooves can amount to between 25 mn2 and 150 mn2. The circumferential grooves can have a width measured in the axial direction of the doctor bar of 0.1 mm to 0.3 mm and a depth of 0.07 mm to 0.13 mm.
An apparatus for coating a material web, especially a paper or cardboard web with a pigment coloring agent, can have two rollers journaled in a support adjacent one another with parallel axes. One roller is fixedly located and the other roller is pressable against the first. At least one applicator and meter system is provided which can be pressed against the surface of one of the rollers outside of the press gap formed by the rollers. The metering system can include as a metering element a doctor bar as has been described.